DESCRIPTION

The EVP cipher routines are a high level interface to certain
symmetric ciphers.

EVP_CIPHER_CTX_init() initializes cipher contex ctx.

EVP_EncryptInit_ex() sets up cipher context ctx for encryption
with cipher type from ENGINE impl. ctx must be initialized
before calling this function. type is normally supplied
by a function such as EVP_des_cbc(). If impl is NULL then the
default implementation is used. key is the symmetric key to use
and iv is the IV to use (if necessary), the actual number of bytes
used for the key and IV depends on the cipher. It is possible to set
all parameters to NULL except type in an initial call and supply
the remaining parameters in subsequent calls, all of which have type
set to NULL. This is done when the default cipher parameters are not
appropriate.

EVP_EncryptUpdate() encrypts inl bytes from the buffer in and
writes the encrypted version to out. This function can be called
multiple times to encrypt successive blocks of data. The amount
of data written depends on the block alignment of the encrypted data:
as a result the amount of data written may be anything from zero bytes
to (inl + cipher_block_size - 1) so out should contain sufficient
room. The actual number of bytes written is placed in outl.

If padding is enabled (the default) then EVP_EncryptFinal_ex() encrypts
the ``final'' data, that is any data that remains in a partial block.
It uses standard block padding (aka PKCS padding). The encrypted
final data is written to out which should have sufficient space for
one cipher block. The number of bytes written is placed in outl. After
this function is called the encryption operation is finished and no further
calls to EVP_EncryptUpdate() should be made.

If padding is disabled then EVP_EncryptFinal_ex() will not encrypt any more
data and it will return an error if any data remains in a partial block:
that is if the total data length is not a multiple of the block size.

EVP_DecryptInit_ex(), EVP_DecryptUpdate() and EVP_DecryptFinal_ex() are the
corresponding decryption operations. EVP_DecryptFinal() will return an
error code if padding is enabled and the final block is not correctly
formatted. The parameters and restrictions are identical to the encryption
operations except that if padding is enabled the decrypted data buffer out
passed to EVP_DecryptUpdate() should have sufficient room for
(inl + cipher_block_size) bytes unless the cipher block size is 1 in
which case inl bytes is sufficient.

EVP_CipherInit_ex(), EVP_CipherUpdate() and EVP_CipherFinal_ex() are
functions that can be used for decryption or encryption. The operation
performed depends on the value of the enc parameter. It should be set
to 1 for encryption, 0 for decryption and -1 to leave the value unchanged
(the actual value of 'enc' being supplied in a previous call).

EVP_CIPHER_CTX_cleanup() clears all information from a cipher context
and free up any allocated memory associate with it. It should be called
after all operations using a cipher are complete so sensitive information
does not remain in memory.

EVP_EncryptInit(), EVP_DecryptInit() and EVP_CipherInit() behave in a
similar way to EVP_EncryptInit_ex(), EVP_DecryptInit_ex and
EVP_CipherInit_ex() except the ctx parameter does not need to be
initialized and they always use the default cipher implementation.

EVP_EncryptFinal(), EVP_DecryptFinal() and EVP_CipherFinal() behave in a
similar way to EVP_EncryptFinal_ex(), EVP_DecryptFinal_ex() and
EVP_CipherFinal_ex() except ctx is automatically cleaned up
after the call.

EVP_get_cipherbyname(), EVP_get_cipherbynid() and EVP_get_cipherbyobj()
return an EVP_CIPHER structure when passed a cipher name, a NID or an
ASN1_OBJECT structure.

EVP_CIPHER_nid() and EVP_CIPHER_CTX_nid() return the NID of a cipher when
passed an EVP_CIPHER or EVP_CIPHER_CTX structure. The actual NID
value is an internal value which may not have a corresponding OBJECT
IDENTIFIER.

EVP_CIPHER_CTX_set_padding() enables or disables padding. By default
encryption operations are padded using standard block padding and the
padding is checked and removed when decrypting. If the pad parameter
is zero then no padding is performed, the total amount of data encrypted
or decrypted must then be a multiple of the block size or an error will
occur.

EVP_CIPHER_key_length() and EVP_CIPHER_CTX_key_length() return the key
length of a cipher when passed an EVP_CIPHER or EVP_CIPHER_CTX
structure. The constant EVP_MAX_KEY_LENGTH is the maximum key length
for all ciphers. Note: although EVP_CIPHER_key_length() is fixed for a
given cipher, the value of EVP_CIPHER_CTX_key_length() may be different
for variable key length ciphers.

EVP_CIPHER_CTX_set_key_length() sets the key length of the cipher ctx.
If the cipher is a fixed length cipher then attempting to set the key
length to any value other than the fixed value is an error.

EVP_CIPHER_iv_length() and EVP_CIPHER_CTX_iv_length() return the IV
length of a cipher when passed an EVP_CIPHER or EVP_CIPHER_CTX.
It will return zero if the cipher does not use an IV. The constant
EVP_MAX_IV_LENGTH is the maximum IV length for all ciphers.

EVP_CIPHER_block_size() and EVP_CIPHER_CTX_block_size() return the block
size of a cipher when passed an EVP_CIPHER or EVP_CIPHER_CTX
structure. The constant EVP_MAX_IV_LENGTH is also the maximum block
length for all ciphers.

EVP_CIPHER_type() and EVP_CIPHER_CTX_type() return the type of the passed
cipher or context. This ``type'' is the actual NID of the cipher OBJECT
IDENTIFIER as such it ignores the cipher parameters and 40 bit RC2 and
128 bit RC2 have the same NID. If the cipher does not have an object
identifier or does not have ASN1 support this function will return
NID_undef.

EVP_CIPHER_CTX_cipher() returns the EVP_CIPHER structure when passed
an EVP_CIPHER_CTX structure.

EVP_CIPHER_mode() and EVP_CIPHER_CTX_mode() return the block cipher mode:
EVP_CIPH_ECB_MODE, EVP_CIPH_CBC_MODE, EVP_CIPH_CFB_MODE or
EVP_CIPH_OFB_MODE. If the cipher is a stream cipher then
EVP_CIPH_STREAM_CIPHER is returned.

EVP_CIPHER_param_to_asn1() sets the AlgorithmIdentifier ``parameter'' based
on the passed cipher. This will typically include any parameters and an
IV. The cipher IV (if any) must be set when this call is made. This call
should be made before the cipher is actually ``used'' (before any
EVP_EncryptUpdate(), EVP_DecryptUpdate() calls for example). This function
may fail if the cipher does not have any ASN1 support.

EVP_CIPHER_asn1_to_param() sets the cipher parameters based on an ASN1
AlgorithmIdentifier ``parameter''. The precise effect depends on the cipher
In the case of RC2, for example, it will set the IV and effective key length.
This function should be called after the base cipher type is set but before
the key is set. For example EVP_CipherInit() will be called with the IV and
key set to NULL,EVP_CIPHER_asn1_to_param() will be called and finally
EVP_CipherInit() again with all parameters except the key set to NULL. It is
possible for this function to fail if the cipher does not have any ASN1 support
or the parameters cannot be set (for example the RC2 effective key length
is not supported.

EVP_CIPHER_CTX_ctrl() allows various cipher specific parameters to be determined
and set.

RETURN VALUES

EVP_EncryptInit_ex(), EVP_EncryptUpdate() and EVP_EncryptFinal_ex()
return 1 for success and 0 for failure.

EVP_DecryptInit_ex() and EVP_DecryptUpdate() return 1 for success and 0 for failure.
EVP_DecryptFinal_ex() returns 0 if the decrypt failed or 1 for success.

EVP_CipherInit_ex() and EVP_CipherUpdate() return 1 for success and 0 for failure.
EVP_CipherFinal_ex() returns 0 for a decryption failure or 1 for success.

EVP_CIPHER_CTX_cleanup() returns 1 for success and 0 for failure.

EVP_get_cipherbyname(), EVP_get_cipherbynid() and EVP_get_cipherbyobj()
return an EVP_CIPHER structure or NULL on error.

RC2 encryption algorithm in CBC, ECB, CFB and OFB modes respectively. This is a variable key
length cipher with an additional parameter called ``effective key bits'' or ``effective key length''.
By default both are set to 128 bits.

EVP_rc2_40_cbc(void), EVP_rc2_64_cbc(void)

RC2 algorithm in CBC mode with a default key length and effective key length of 40 and 64 bits.
These are obsolete and new code should use EVP_rc2_cbc(), EVP_CIPHER_CTX_set_key_length() and
EVP_CIPHER_CTX_ctrl() to set the key length and effective key length.

RC5 encryption algorithm in CBC, ECB, CFB and OFB modes respectively. This is a variable key length
cipher with an additional ``number of rounds'' parameter. By default the key length is set to 128
bits and 12 rounds.

GCM Mode

For GCM mode ciphers the behaviour of the EVP interface is subtly altered and
several GCM specific ctrl operations are supported.

To specify any additional authenticated data (AAD) a call to EVP_CipherUpdate(),
EVP_EncryptUpdate() or EVP_DecryptUpdate() should be made with the output
parameter out set to NULL.

When decrypting the return value of EVP_DecryptFinal() or EVP_CipherFinal()
indicates if the operation was successful. If it does not indicate success
the authentication operation has failed and any output data MUST NOT
be used as it is corrupted.

The following ctrls are supported in GCM mode:

EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_SET_IVLEN, ivlen, NULL);

Sets the GCM IV length: this call can only be made before specifying an IV. If
not called a default IV length is used (96 bits for AES).

EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_GET_TAG, taglen, tag);

Writes taglen bytes of the tag value to the buffer indicated by tag.
This call can only be made when encrypting data and after all data has been
processed (e.g. after an EVP_EncryptFinal() call).

EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_SET_TAG, taglen, tag);

Sets the expected tag to taglen bytes from tag. This call is only legal
when decrypting data and must be made before any data is processed (e.g.
before any EVP_DecryptUpdate() call).

See EXAMPLES below for an example of the use of GCM mode.

CCM Mode

The behaviour of CCM mode ciphers is similar to CCM mode but with a few
additional requirements and different ctrl values.

Like GCM mode any additional authenticated data (AAD) is passed by calling
EVP_CipherUpdate(), EVP_EncryptUpdate() or EVP_DecryptUpdate() with the output
parameter out set to NULL. Additionally the total plaintext or ciphertext
length MUST be passed to EVP_CipherUpdate(), EVP_EncryptUpdate() or
EVP_DecryptUpdate() with the output and input parameters (in and out)
set to NULL and the length passed in the inl parameter.

The following ctrls are supported in CCM mode:

EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_CCM_SET_TAG, taglen, tag);

This call is made to set the expected CCM tag value when decrypting or
the length of the tag (with the tag parameter set to NULL) when encrypting.
The tag length is often referred to as M. If not set a default value is
used (12 for AES).

EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_CCM_SET_L, ivlen, NULL);

Sets the CCM L value. If not set a default is used (8 for AES).

EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_CCM_SET_IVLEN, ivlen, NULL);

Sets the CCM nonce (IV) length: this call can only be made before specifying
an nonce value. The nonce length is given by 15 - L so it is 7 by default
for AES.

NOTES

Where possible the EVP interface to symmetric ciphers should be used in
preference to the low level interfaces. This is because the code then becomes
transparent to the cipher used and much more flexible. Additionally, the
EVP interface will ensure the use of platform specific cryptographic
acceleration such as AES-NI (the low level interfaces do not provide the
guarantee).

PKCS padding works by adding n padding bytes of value n to make the total
length of the encrypted data a multiple of the block size. Padding is always
added so if the data is already a multiple of the block size n will equal
the block size. For example if the block size is 8 and 11 bytes are to be
encrypted then 5 padding bytes of value 5 will be added.

When decrypting the final block is checked to see if it has the correct form.

Although the decryption operation can produce an error if padding is enabled,
it is not a strong test that the input data or key is correct. A random block
has better than 1 in 256 chance of being of the correct format and problems with
the input data earlier on will not produce a final decrypt error.

If padding is disabled then the decryption operation will always succeed if
the total amount of data decrypted is a multiple of the block size.

The functions EVP_EncryptInit(), EVP_EncryptFinal(), EVP_DecryptInit(),
EVP_CipherInit() and EVP_CipherFinal() are obsolete but are retained for
compatibility with existing code. New code should use EVP_EncryptInit_ex(),
EVP_EncryptFinal_ex(), EVP_DecryptInit_ex(), EVP_DecryptFinal_ex(),
EVP_CipherInit_ex() and EVP_CipherFinal_ex() because they can reuse an
existing context without allocating and freeing it up on each call.

BUGS

For RC5 the number of rounds can currently only be set to 8, 12 or 16. This is
a limitation of the current RC5 code rather than the EVP interface.

EVP_MAX_KEY_LENGTH and EVP_MAX_IV_LENGTH only refer to the internal ciphers with
default key lengths. If custom ciphers exceed these values the results are
unpredictable. This is because it has become standard practice to define a
generic key as a fixed unsigned char array containing EVP_MAX_KEY_LENGTH bytes.

The ASN1 code is incomplete (and sometimes inaccurate) it has only been tested
for certain common S/MIME ciphers (RC2, DES, triple DES) in CBC mode.